Generator backup and PV, no batteries

ggunnggunn PV EngineerPosts: 1,972Solar Expert
This question has come up twice in the past couple of days. Potential customers want PV for their house and a generator backup for when the grid goes down, and they want the generator to furnish the grid to the PV inverter so that it keeps running. I see a problem with backfeeding the generator in the case where the inverter output exceeds the load.

The safe way to do this, of course, would be to put the inverter outside the transfer switch so that it shuts down when the grid goes down, but that's not what they want. They want their PV to keep running when the grid goes down, and they are not interested in spending the money to get a Sunny Island and batteries.

Is there a safe and cost effective way to do this?
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Comments

  • RCinFLARCinFLA New User Posts: 1,280Solar Expert
    Re: Generator backup and PV, no batteries

    Depend on your definition of cost effective.

    An XW inverter will do this. With latest software you can connect a grid-tie inverter to its output.
  • ggunnggunn PV Engineer Posts: 1,972Solar Expert
    Re: Generator backup and PV, no batteries
    RCinFLA wrote: »
    Depend on your definition of cost effective.

    An XW inverter will do this. With latest software you can connect a grid-tie inverter to its output.

    Aren't XW inverters battery inverters? These guys are not interested in batteries or extra inverters.
  • Peter_VPeter_V Solar Experimenter Posts: 212Solar Expert
    Re: Generator backup and PV, no batteries
    ggunn wrote: »
    This question has come up twice in the past couple of days. Potential customers want PV for their house and a generator backup for when the grid goes down, and they want the generator to furnish the grid to the PV inverter so that it keeps running. I see a problem with backfeeding the generator in the case where the inverter output exceeds the load.

    The safe way to do this, of course, would be to put the inverter outside the transfer switch so that it shuts down when the grid goes down, but that's not what they want. They want their PV to keep running when the grid goes down, and they are not interested in spending the money to get a Sunny Island and batteries.

    Is there a safe and cost effective way to do this?

    Probably not a problem. Most GT inverters will cut out when the voltage exceeds a certain level, this level is generally less than the max the voltage the generators can handle.

    I know several people around here who have run Enphase micro-inverters on generator power. Normally what happens is the generator just runs at idle to provide "grid" power/frequency and the micro-inverters handle the load, until the load exceeds the arrays power and then the generator ramps up to handle the rest.

    The micro-inverters cut out at about 264V, but there is enough variation between inverters that I suspect they drop out one at a time if the load is less than your array's output. It takes a few seconds for them to turn back on when the load increases, but the generator handles that.

    How other inverters handle running of generator will probably depend on the inverter. If the inverter scales back power out when line voltage goes up, you're good to go. If however if it produces full power until it hits max voltage and then shuts down, then it's not going to work very well. The inverter will keep turning on and off and likely something will fail.

    Also you'll need a generator that produces a very stable frequency. Many inverters will shut down if the frequency varies from normal by more than 0.5 hz (50 or 60hz depending on where you live)
  • ggunnggunn PV Engineer Posts: 1,972Solar Expert
    Re: Generator backup and PV, no batteries

    I talked to SMA tech support. The guy I talked to said that he gets asked this question on average twice a week, and the answer is "don't do it". He said not to worry about the Sunny Boy, it's perfectly capable of protecting itself, but the control board of a generator is not so robust. He said that the line voltage may rise enough to shut off the Sunny Boy when the load consumption drops below the output of the SB, but the control board may very well be dead by the time that happens.
  • techntrektechntrek I trek with tech Posts: 1,355Solar Expert ✭✭
    Re: Generator backup and PV, no batteries
    ggunn wrote: »
    Aren't XW inverters battery inverters? These guys are not interested in batteries or extra inverters.

    It does both - it can grid-tie, be off-grid, or do some combination of both (either run from batteries until they are below a certain point and then use the grid, or act as a battery backup to the grid if it goes down). But, it doesn't do it as efficiently as a dedicated grid-tie inverter like the Sunny Boy. So you trade a lot of output over time for the additional ability if they don't want more than one inverter.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • ggunnggunn PV Engineer Posts: 1,972Solar Expert
    Re: Generator backup and PV, no batteries
    techntrek wrote: »
    It does both - it can grid-tie, be off-grid, or do some combination of both (either run from batteries until they are below a certain point and then use the grid, or act as a battery backup to the grid if it goes down). But, it doesn't do it as efficiently as a dedicated grid-tie inverter like the Sunny Boy. So you trade a lot of output over time for the additional ability if they don't want more than one inverter.
    But the question is, can it mediate between PV, grid, and generator without batteries? Can it keep the PV contributing to the load when the grid is down and still protect the generator from getting backfed? Or what if you installed one with a very small battery bank that would virtually always be too little to power the loads?

    Gensets are a lot cheaper than batteries. If there were a way to keep PV running in the event of an outage without having to shell out thousands for batteries and extra inverters, a lot of folks would be interested, especially in hurricane prone areas. A friend of mine who lives just outside the loop in Houston was without power for seven weeks after Hurricane Ike.
  • BB.BB. Just some guy Posts: 24,213Super Moderators admin
    Re: Generator backup and PV, no batteries

    No, the Xantrex XW hybrid inverter needs the batteries... And batteries are still the expensive/weak point/need replacing over time too which drives up the $$/kWH price of the power produced.

    From my point of view, it is still a better solution (hybrid GT/Off Grid inverter) than a pure Off-Grid system. The ability to use grid for the 99% of the time the grid is up allows more energy to be used (over time; and more efficient/less battery losses) and less cycling stresses on the batteries--leading to a better $$/kWH price.

    However, at this point, there is no "cheap" solution for emergency power other than the genset (which can become very expensive to supply fuel for if there is a prolonged outage).

    Conservation so that you have the smallest possible loads and the smallest genset possible to run those loads can reduce fuel usage and storage needs by a whole lot.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunnggunn PV Engineer Posts: 1,972Solar Expert
    Re: Generator backup and PV, no batteries
    BB. wrote: »
    No, the Xantrex XW hybrid inverter needs the batteries... And batteries are still the expensive/weak point/need replacing over time too which drives up the $$/kWH price of the power produced.

    From my point of view, it is still a better solution (hybrid GT/Off Grid inverter) than a pure Off-Grid system. The ability to use grid for the 99% of the time the grid is up allows more energy to be used (over time; and more efficient/less battery losses) and less cycling stresses on the batteries--leading to a better $$/kWH price.

    However, at this point, there is no "cheap" solution for emergency power other than the genset (which can become very expensive to supply fuel for if there is a prolonged outage).

    Conservation so that you have the smallest possible loads and the smallest genset possible to run those loads can reduce fuel usage and storage needs by a whole lot.
    Tha's what I thought and I agree, but what we are being asked to supply is PV to offset the grid charges when the grid is up (and sell back to the grid when production exceeds usage), and power the loads from a combination of PV and generator power when the grid is down without having to buy batteries and another inverter. What they are asking for is a way around anti-islanding by substituting generator power for grid power to the inverter. I don't know of a way to do that without putting the genset at risk if the power consumed by the load falls below the output of the inverter.
  • BB.BB. Just some guy Posts: 24,213Super Moderators admin
    Re: Generator backup and PV, no batteries

    In general, I am not sure having solar "GT" supporting a genset is really the best use of technology and $$$...

    If you use solar GT to off set the generator, you are "unloading" the genset... And, as a very rough rule of thumb, unloading the genset below ~50% of rated load, you are saving very little fuel (and with diesels, you run the risk of "coking", "wet stack", "glazing the cylinder walls", etc.).

    My own two cents, would be to run a "Hybrid" (Grid Tied/Off Grid--if GT is supported by your utility) or the Xantrex/Schneider XW Hybrid inverter (with generator assist--possibly has the option for "grid assist" where Grid Tied is not allowed). And use set the system to load the genset >50% rated load while charging the battery bank.

    You only run the genset when needed and you run it in its fuel/mechanically efficient power band with the minimum run-time necessary to keep your batteries up (when loads exceed sunlight).

    Cycling batteries does wear them out faster--but with proper charging/cycling, you can still get many good years (or even a decade or more) with the batteries cycling.

    Without going into a whole bunch of assumption driven math--I would have to believe that running a "hybrid system" (solar PV + hybrid inverter + genset running >50% rated load to charge battery bank/support AC loads while charging) is going to be better/more cost efficient than trying to use GT solar (without battery bank) trying to "support" a generator by the time fuel costs and the 3,000-10,000 hour typical geset life.

    If we want to make some assumptions (generator size, fuel flow, costs,life, battery bank costs/life, efficiencies, amount of backup power needed per year based on primary genset vs primary hybrid inverter, etc.), we could try some simple $$$/kWH math and see what falls out.

    I would not be surprised if the costs all came out to a "wash" (similar $$$/kWH costs) for most of the various options (with one or two being way more costly).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Solar GuppySolar Guppy Ex Solar Engineer Posts: 1,959Solar Expert
    Re: Generator backup and PV, no batteries
    ggunn wrote: »
    Tha's what I thought and I agree, but what we are being asked to supply is PV to offset the grid charges when the grid is up (and sell back to the grid when production exceeds usage), and power the loads from a combination of PV and generator power when the grid is down without having to buy batteries and another inverter. What they are asking for is a way around anti-islanding by substituting generator power for grid power to the inverter. I don't know of a way to do that without putting the genset at risk if the power consumed by the load falls below the output of the inverter.

    And I assume if your doing this for hire, so simply tell the client the technology does meet their requirements and end of story.

    Trying to sell a non-ul listed solution is potentially business ending proposition and one can bet the user is somehow going to have have the generator back feeding the grid at some point and when they go to find out why, home owner will say, "Oh we can do it solar installed in that way!"
  • ggunnggunn PV Engineer Posts: 1,972Solar Expert
    Re: Generator backup and PV, no batteries
    And I assume if your doing this for hire, so simply tell the client the technology does meet their requirements and end of story.
    I assume that you meant to write "does not meet their requirements", and that may very well be what I tell them if I find it to be the case after thoroughly checking it out.
    Trying to sell a non-ul listed solution is potentially business ending proposition and one can bet the user is somehow going to have have the generator back feeding the grid at some point and when they go to find out why, home owner will say, "Oh we can do it solar installed in that way!"

    Backfeeding the grid from the genset isn't a concern because in any case a transfer switch would always keep them separated, but of course I will not be selling/installing anything that isn't UL listed for the application at hand. I just need to investigate it thoroughly before I punt.
  • techntrektechntrek I trek with tech Posts: 1,355Solar Expert ✭✭
    Re: Generator backup and PV, no batteries
    ggunn wrote: »
    Gensets are a lot cheaper than batteries.

    Don't make the same mistake many others make. Gensets are a lot cheaper to buy than batteries, for the same output. Gensets are expensive to run and maintain. So when you weigh PV vs. genset, you have to weigh the total system cost over time. I wish someone told me this before I bought my genset...
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • ggunnggunn PV Engineer Posts: 1,972Solar Expert
    Re: Generator backup and PV, no batteries
    techntrek wrote: »
    Don't make the same mistake many others make. Gensets are a lot cheaper to buy than batteries, for the same output. Gensets are expensive to run and maintain. So when you weigh PV vs. genset, you have to weigh the total system cost over time. I wish someone told me this before I bought my genset...

    Granted, but in a backup situation the big unknown is how much the backup, whether batteries or a genset, will be used. The folks we are talking to are betting that it won't be used much.
  • RCinFLARCinFLA New User Posts: 1,280Solar Expert
    Re: Generator backup and PV, no batteries

    You could line tap the GT inverter then have a regular transfer switch between grid line input and generator. Transfer switch must handle the rated service amperage so it won't be cheap.

    If they only want partial backup you can put a lower amperage transfer switch to an auxilary box.

    A transfer switch will not be uninterruptable service like XW w/ batteries. It will drop power until generator starts up and comes online.
  • ggunnggunn PV Engineer Posts: 1,972Solar Expert
    Re: Generator backup and PV, no batteries
    RCinFLA wrote: »
    You could line tap the GT inverter then have a regular transfer switch between grid line input and generator. Transfer switch must handle the rated service amperage so it won't be cheap.

    If they only want partial backup you can put a lower amperage transfer switch to an auxilary box.

    A transfer switch will not be uninterruptable service like XW w/ batteries. It will drop power until generator starts up and comes online.

    I realize that, but the arrangement you suggest would shut down the PV inverter in the event of a grid outage if the tap is outside the transfer switch, which is what they are trying to get around. If the tap is between the transfer switch and the load, then the generator is vulnerable to backfeed from the PV inverter if the power consumed by the load falls below the output of the inverter, which is what I want to avoid. Hence the conundrum.
  • bobdogbobdog Gila Manster Posts: 191Solar Expert ✭✭✭✭
    Re: Generator backup and PV, no batteries

    Gonna hijack for a second.

    BB (Bill) you mention in your signature that you are GT with a generator for emergency back up. How do you use your generator? Is it somehow tied into your GT system, or do you simply use extension cords? Thanks and sorry for bringing up and old thread with a hijack.

    Tim
  • BB.BB. Just some guy Posts: 24,213Super Moderators admin
    Re: Generator backup and PV, no batteries

    Just a cheap six circuit manual transfer switch wired up to only support A 120 volt genset.

    My gt system would require a 240/120 volt generator.
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bobdogbobdog Gila Manster Posts: 191Solar Expert ✭✭✭✭
    Re: Generator backup and PV, no batteries
    BB. wrote: »
    Just a cheap six circuit manual transfer switch wired up to only support A 120 volt genset.

    My gt system would require a 240/120 volt generator.
    -Bill

    Thanks, that makes sense.

    Tim
  • System2System2 Posts: 6,290 admin
    Re: Generator backup and PV, no batteries

    I am also interested in such a system. One where I can take advantage of the PV generated power during an outage, yet not have a large cost tied up to support a rare event.

    99% of the time GT solar is what I need, every once and a while the power goes out. It is a bit silly to be sitting on 2kw of generating capacity and not be able to use it. I don't need instant UPS transfer ability or the ability to run everything 24 hours a day, even a manual system would be fine where I could limit the circuits to say the fridg/freezer and a few lights. Extension cords are even an option.

    The trouble seems to be that batteries are so expensive and have a limited lifespan. There also is a relationship on the possible power out of a GT inverter and input voltage. The 48v inverters give the best expandable capacity and most power out yet require the most batteries. It is hard to justify a $3k investment in batteries with limited life for a 1% event.

    Yet there must be a way to configure a system to minimize the investment in batteries, charge controllers and gensets and still receive some of the benefit from the capital investment on the inverter and PV side. I also think an affordable solution to this problem would make solar attractive to a larger audience.

    Rearden
  • ggunnggunn PV Engineer Posts: 1,972Solar Expert
    Re: Generator backup and PV, no batteries
    rearden wrote: »
    I am also interested in such a system. One where I can take advantage of the PV generated power during an outage, yet not have a large cost tied up to support a rare event.

    99% of the time GT solar is what I need, every once and a while the power goes out. It is a bit silly to be sitting on 2kw of generating capacity and not be able to use it. I don't need instant UPS transfer ability or the ability to run everything 24 hours a day, even a manual system would be fine where I could limit the circuits to say the fridg/freezer and a few lights. Extension cords are even an option.

    The trouble seems to be that batteries are so expensive and have a limited lifespan. There also is a relationship on the possible power out of a GT inverter and input voltage. The 48v inverters give the best expandable capacity and most power out yet require the most batteries. It is hard to justify a $3k investment in batteries with limited life for a 1% event.

    Yet there must be a way to configure a system to minimize the investment in batteries, charge controllers and gensets and still receive some of the benefit from the capital investment on the inverter and PV side. I also think an affordable solution to this problem would make solar attractive to a larger audience.

    Rearden
    There are several reasons why this will not work, even if you could get around the UL1741 requirement that any grid tied inverter shut itself off when the grid goes down (this so that your inverter does not energize the power lines outside your home and potentially electrocute utility line workers who are out there trying to get you back on line).

    The primary one is that the power from a PV array is not a constant thing. Clouds, birds, planes passing over, the varying strength and orientation of the sun as it traverses the sky, temperature variations, etc... these kinds of things can drastically and suddenly affect the power that an array is producing at any given moment. Household loads don't track these events, so when the sun goes away and you have your A/C running, it's going to be starved for power. The lights you have on will dim or go out. Some equipment could be damaged.

    A battery bank serves to buffer the output of a PV system so that your loads can draw from a constant source of power while the output from your array is all over the map. There really is no way around it. As Scotty used to say, "I cannae rewrite the laws of physics, Cap'n!"
  • stephendvstephendv New User Posts: 1,571Solar Expert
    Re: Generator backup and PV, no batteries

    It could be possible to DIY an inverter-generator from mostly off the shelf the components and then rig it so that you AC couple grid connected inverters to the output. Would be interested in hearing your views...

    What I'm thinking is an engine connected to a powerful 12V alternator, say 200A connected to a 12V car battery this could produce up to 2kW. Then connect a basic 2kW bidirectional inverter/charger to the battery and a dump controller + dump load to the battery. The dump load should also be rated at 2kW. Then connect the grid tied inverter to the output of the inverter/charger in the classic AC backfeeding configuration.

    The tricky bit will be to build an electronic governor that changes the engine speed based on the current entering the dump load. The battery won't be cycled at all, it just acts a buffer for the seconds it would take the governor to speed up the engine.

    So for example, if the loads are 1kW and the PV can produce 2kW, then all 2kW are produced, a small portion of it will be used to charge the battery and the rest goes to the dump load. The governor detects the current going to the dump load and reduces the engine speed to idle.

    Then if there are 4kW loads and only 2kW from the panels there will be nothing going to the dump load, so the governor will act in the opposite direction and open the engine to full RPM in order to deliver the full power from the alternator. 2kW from the alternator which goes through the battery inverter/charger + 2kW from the AC connected panels.

    The limitations would be that the amount of connected PV is limited by the size of the inverter/charger: if it can only charge with 2kW then you can only connect 2kW of panels (is there a way to resolve this? what happens if you stick 4kW of PV backfeeding a 2kW inverter/charger?). Also, once you add up all the costs, I don't know if it'll end up being comparable to just buying more batteries and an XW or sunny island setup...?

    Thoughts?
  • ggunnggunn PV Engineer Posts: 1,972Solar Expert
    Re: Generator backup and PV, no batteries
    stephendv wrote: »
    It could be possible to DIY an inverter-generator from mostly off the shelf the components and then rig it so that you AC couple grid connected inverters to the output. Would be interested in hearing your views...

    What I'm thinking is an engine connected to a powerful 12V alternator, say 200A connected to a 12V car battery this could produce up to 2kW. Then connect a basic 2kW bidirectional inverter/charger to the battery and a dump controller + dump load to the battery. The dump load should also be rated at 2kW. Then connect the grid tied inverter to the output of the inverter/charger in the classic AC backfeeding configuration.

    The tricky bit will be to build an electronic governor that changes the engine speed based on the current entering the dump load. The battery won't be cycled at all, it just acts a buffer for the seconds it would take the governor to speed up the engine.

    So for example, if the loads are 1kW and the PV can produce 2kW, then all 2kW are produced, a small portion of it will be used to charge the battery and the rest goes to the dump load. The governor detects the current going to the dump load and reduces the engine speed to idle.

    Then if there are 4kW loads and only 2kW from the panels there will be nothing going to the dump load, so the governor will act in the opposite direction and open the engine to full RPM in order to deliver the full power from the alternator. 2kW from the alternator which goes through the battery inverter/charger + 2kW from the AC connected panels.

    The limitations would be that the amount of connected PV is limited by the size of the inverter/charger: if it can only charge with 2kW then you can only connect 2kW of panels (is there a way to resolve this? what happens if you stick 4kW of PV backfeeding a 2kW inverter/charger?). Also, once you add up all the costs, I don't know if it'll end up being comparable to just buying more batteries and an XW or sunny island setup...?

    Thoughts?
    It seems to me that if you are going to have that motor running all the time anyway, you might as well just run off a generator and forget about the PV.
  • stephendvstephendv New User Posts: 1,571Solar Expert
    Re: Generator backup and PV, no batteries
    ggunn wrote: »
    It seems to me that if you are going to have that motor running all the time anyway, you might as well just run off a generator and forget about the PV.

    The big difference would be fuel consumption. A motor idling along just enough to provide an AC reference will consume much much less than one delivering it's rated output all the time.
  • CariboocootCariboocoot Gone Fishing... Posts: 17,615Banned
    Re: Generator backup and PV, no batteries

    stephendv;

    Have you looked at the cost of doing this? Unless you have the stuff laying around or can get some really good used deals you run in to this:
    Exeltech 2kW 12 Volt inverter: $1,277.00
    Honda EU2000i inverter-generator: $1,079.95

    Sort of makes it impractical, doesn't it?
  • stephendvstephendv New User Posts: 1,571Solar Expert
    Re: Generator backup and PV, no batteries

    Yeaah, price, hrmmmf. :cry:
  • ggunnggunn PV Engineer Posts: 1,972Solar Expert
    Re: Generator backup and PV, no batteries
    stephendv wrote: »
    The big difference would be fuel consumption. A motor idling along just enough to provide an AC reference will consume much much less than one delivering it's rated output all the time.
    Yes, but how long would it take to burn the difference in the amount of fuel that would offset what the PV system would have cost? My guess is "a long time".
  • techntrektechntrek I trek with tech Posts: 1,355Solar Expert ✭✭
    Re: Generator backup and PV, no batteries

    Remember that an engine running at idle burns about the same fuel as one at half load, you don't get into an efficient burn until you load it most of the way. Having an engine idling along all the time would be terribly inefficient.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • stephendvstephendv New User Posts: 1,571Solar Expert
    Re: Generator backup and PV, no batteries
    techntrek wrote: »
    Remember that an engine running at idle burns about the same fuel as one at half load, you don't get into an efficient burn until you load it most of the way. Having an engine idling along all the time would be terribly inefficient.

    It'll only be inefficient if it idles at the same RPM as when loaded, which is what AC gens have to do since even if they're idling they're still expected to produce 60Hz. But if you used a DC alternator and DIY governor then you could idle at a much lower RPM and save fuel.

    ggunn, the OP was asking about a backup system for his PV. So the cost comparison would be between:
    a) Crazy DIY DC generator idea
    b) batteries + battery Inverter
    c) AC generator like honda EU

    As 'coot was saying option c seems to make the most economic sense.
  • debraredheaddebraredhead New User Posts: 1Registered Users
    Re: Generator backup and PV, no batteries

    I have a specific situation that relates closely with this thread. My bf and I were able to purchase a used 5 kw Solar Edge (SE) inverter and power boxes along with the used panels from a friend losing her house.

    We want to use them on a cabin we are re-building in the desert in Southern California. The cabin is off grid. We plan on working on it during the daylight hours only for about the next year or so. Our loads will be simple. A small fridge to keep drinks and condiments cool (it can be off at night), a radio, a laptop (a Dell that no longer charges the battery), a couple of chargers for battery drills etc. and to run one power tool at a time.

    We spent about 10x the amount on the solar system than we were going to spend on a gas generator, but couldn’t pass it up since we would have lost our deal by the time we need a complete off grid system.

    From reading this thread I can see a few problems for us, namely:

    1. Our SE inverter will not turn on because there is no grid. (not a safety issue for us but a practical one non-the-less)

    2. Most of the time our loads will be much less than the power that the PV system will be generating.

    3. Possible damage to our equipment if our load surpasses PV output.

    The thing we have going for us that is probably not normal, is that we are not worried about temporary disruptions of power (except if out equipment is ruined).

    I read in this thread about “dump load”. I assume this refers to some type of circuit that will somehow get rid of any excess power generated. Is this necessary and if so how can we make one?

    Turning on the SE inverter. Is it possible to use a little 250 w 12v/120 v inverter I own by powering it with a car battery and hooking it up to the AC side of the SE inverter? (mine is not a pure sine wave). If it does fake out the SE inverter to turn on, will the power now coming from the PV damage the small inverter? If I had to buy a small inverter what spec would allow for the PV power supply to be added without damage? What spec would fake out the SE inverter? Would I need two small inverters, one for each L1 and L2 of the SE inverter, or would one be able to fake out both lines? Or is the monitoring for grid connect only done by one of the Lines?

    Protecting from damage. I assume that if the PV generation falls, our loads would place a high demand on the small 250w inverter causing it to overload and shut down. The SE inverter would then sense no “grid” and shut down saving our equipment from damage.

    Are we missing something or do you think this will work for our situation?
  • CariboocootCariboocoot Gone Fishing... Posts: 17,615Banned
    Re: Generator backup and PV, no batteries
    Are we missing something or do you think this will work for our situation?

    Welcome to the forum.

    Sorry to say your plan won't work. The first problem being the not sine wave inverter will create an output waveform that the GT inverter will see as "no or bad grid". As far as the Solar Edge is concerned the Voltage (and it needs 240 so the output from the small inverter would have to go through a step-up transformer) and frequency would be wrong. The chances of it starting are pretty slim.

    Some people have got GT inverters to start using small pure sine inverters running off batteries. The problem is regulating the power, as GT inverters try to push line Voltage up. Normally the electrical inertia of the grid keeps the Voltage down, and the difference creates the current flow. With a small off-grid inverter as the sync source the GT inverter can send the Voltage high. Depending on the exact set-up, this either burns up the little inverter or tries to charge the battery through it or causes the GT to shut down when in notices line Voltage has gone too high.

    The middle scenario there is the one that can be worked with. Certain inverters are designed to do this and have some regulatory system built in. Most notably the Sunny Island and Xantrex XW series. That's $5,000 and $3,000. Doesn't sound like a practical solution, does it? You might be able to get another type to co-operate, but since they aren't designed to work this way the results are hit-or-miss, with emphasis on the 'miss' and it potentially catastrophic results.

    My advice would be if you're going for off-grid anyway, sell the Solar Edge and put the money towards the right kind of equipment to begin with. The other route is strictly for experimenters with masochistic tendencies.
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